For certain applications, it is necessary to synchronize the operation of a pair of stepped synthesizers at stations that communicate with each other via a transmission link. To enable a single beat frequency to be processed at a receiving station, the synthesizers are desirably stepped in synchronism with each other so that when a step in the frequency of the wave received at a receiving station occurs, the frequency of the synthesizer at the receiving station is simultaneously stepped.
In one particular application, such synthesizers are employed in portable or fixed calibrating stations that transmit radio waves having predetermined frequencies and amplitudes through a relay station on a geostationary, synchronous satellite, to a ground station having a synthesizer similar to that of the calibration station. The synthesizer at the ground station must be stepped in synchronism with the occurrence times of frequency steps in the wave received from the calibration stations in order to derive a constant beat frequency that can be passed to an IF amplifier. Because of the significant delay time of propagation between the stations (e.g., about 0.25 seconds) time reference signals, such as from WWV, cannot be employed to control the synthesizers. For this particular application, the synthesizers at the calibration stations cause the transmitted wave to be stepped in 100 kHz intervals over a 0.5 gHz frequency band, centered approximately at 6 gHz. Because of these spectral requirements, it is necessary to eliminate all self-interference between the signals transmitted between the stations.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for synchronizing the stepping of synthesizers at stations that communicate with each other.
Another object of the invention is to provide apparatus for synchronizing synthesizers at first and second stations that communicate with each other via a transmission link so that frequencies which are sequentially derived from a transmitting station are received at a receiving station where the synchronizer is stepped in synchronism with changes in the received frequencies so as to enable a constant beat frequency to be derived by mixing the received wave and a wave from the receiving station synthesizer.
A further object of the invention is to provide a relatively inexpensive apparatus for synchronizing a synthesizer at a receiving station with steps in the frequency of a wave received at the station.